Our main objective is to study rates of biochemical evolution, and its relationship to rates of morphological change. We will do this by comparing pairs of populations of tropical Atlantic and Pacific new world fishes which have been separated for several million years by the closure of the isthmus of Panama. Although the pairs have been separated for approximately the same time, morphological differences range from undetectable to the level of the species. Such species pairs are a conspicuous element of the fish fauna and have been termed geminate species. The degree to which genetic changes are correlated with morphological changes will be examined. Genetically controlled differences in enzyme systems will be determined by detection of isozymes with starch-gel electrophoresis. We will be able to determine whether genetic differentiation at the biochemical level proceeds at a constant rate, and thus test the hypothesis of an evolutionary clock.